Atomically thin graphene is a promising capacitor electrode material, but the cell voltages achieved so far with various electrolytes are too low for a typical capacitor's application. In this paper it is demonstrated that in a dicyanamide-based ionic liquid electrolyte a graphene electrode passivates at 10 V and at the same time the conductivity of the single-layer graphene sheet increases more than 5 times. The protective self-healing nanolayer formed is a new carbon nitrogen material with dielectric constant value up to 9.8. In the cell voltage range from 4 to 10 V, capacitors with large surface area reduced graphene oxide electrodes outperform any commercial capacitor technology by better energy density. (C) 2015 Elsevier B.V. All rights reserved.